A polymer may be deposited on various substrates, such as glass, plastics, metals, and polymers, using chemical vapor deposition (CVD) techniques, which include plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), hot-wire chemical vapor deposition (HWCVD), and initiated chemical vapor deposition (iCVD) techniques.
In CVD, monomers are converted directly to desired polymeric films without the need for purification, drying, or curing steps. Custom copolymers can be created simply by changing the ratio of feed gases to the CVD reactor (Murthy, S. K.; Gleason, K. K. Macromolecules 2002, 35, 1967). CVD allows films of nanoscale thicknesses with macroscale uniformity to be produced, and the method can be applied to complex geometries. (Pierson, H. O. Handbook of Chemical Vapor Deposition, 2nd ed.; Noyes Publications: Norwich, N.Y., 1999). CVD can also be used to coat nanoscale features, as the technique is not subject to surface-tension and non-uniform-wetting effects that are typically associated with wet processes.
Protective coatings, which prevent the permeation of water into organic optoelectronic devices, including organic photovoltaic (OPV) devices fabricated on flexible plastic substrates, are essential to extend device lifetimes. (M. S. Weaver et al., Appl. Phys. Lett., 2002, 81, 2929). Widely investigated barrier protective coatings are made of multilayer stacks, wherein multiple dense, inorganic layers are alternated with soft, organic layers. Triads have shown water vapor transmission rates (WVTR) less than 10−4 g/cm2/day.
Even though the permeability coefficient of a single thin inorganic layer of silicon dioxide or aluminum oxide would in theory be low enough to allow them to serve as perfect barriers, residual permeation through them is always detected. The experimentally observed permeability is due to the presence of unwanted but inevitable nano-, microscopic defects and pinholes that limit the minimum WVTR achievable by a single inorganic layer to 10−2 g/m2/day. (Weaver, 2002; A. S. da Silva Sobrinho et al., J. Vac. Sci. Technol. A, 2000, 18, 2021). The pinholes may result from the presence of dust particles on the substrate surface during deposition, from geometric shadowing and stress during film growth at sites of high surface roughness, or from powder formation in the plasma phase during deposition. These defects lead to oxidation and corresponding shorter device lifetimes. The role of the organic layer is (i) decoupling the defects among two successive inorganic layers, thus forcing the permeant molecules to follow a tortuous and longer path (G. L. Graff et al., J. Appl. Phys., 2004, 96, 1840); (ii) filling the pores of the inorganic underlayer, limiting the propagation of defects from one inorganic layer to the other (A. G. Erlat et al., J. Phys. Chem. B, 1999, 103, 6047); and (iii) smoothening the substrate surface roughness and covering dust or anti-blocking particles on the surface. (P. E. Burrows et al., Displays, 2001, 22, 65).